Lawn mower is a garden machine which is specially used for mowing lawns. Typically, it comprises a frame, a mower deck, an engine, a driving mechanism, wheels, blades, steering means as well as other control components. High speed blades are provided within the mower deck, and a grass discharge opening is typically provided in a side wall of the mower deck, and the grass discharge opening is provided with a cover thereon. At the bottom of the mower deck, mower deck supporting wheels are generally mounted. The mower deck moves along with the frame and the running wheels, and the height of the mower deck determines the height of mowing, i.e., only the grass higher than the blades will be cut. The mower deck of a conventional lawn mower will be suspended from the frame via a chain, and the mowing height may be varied by adjusting the position of a height limit pin. The efficiency of such a structure in adjusting the height of the mower deck is low, and the mower deck tends to waggle when the lawn mower is running or changing speed. Thus, the mower deck as a whole is not sufficiently stable, which would affect the flatness of mowing.